1. Field of the Invention
The present invention relates to a numerical control device for controlling a five-axis processing machine that processes a workpiece attached to a table by the use of three linear axes and two rotation axes.
2. Description of the Related Art
In a case where a workpiece is processed by a five-axis processing machine, a processing technique is generally used which also interpolates a tool direction while interpolating a tool center point movement path based on an instructed relative movement speed between the workpiece and a tool with respect to a movement instruction of a tool center point in the movement path and the tool direction, and causes the tool center point to move along an instructed movement path at an instructed speed while changing the tool direction. Such an instruction and a processing technique are called a tool center point control, and a program instruction is generally created by CAM.
JP-A 2003-195917 discloses a technique which corrects a movement path interpolation point while interpolating a movement path and a tool direction of a tool center point based on a relative movement speed between a workpiece and a tool with respect to the movement instruction of the tool center point and drives a servo motor so that the tool center point, moves along an instructed movement path at an instructed speed.
JP-A 2005-182437 discloses a technique which interpolates a curve in which a processing point is generated from an instruction point sequence so that the curve changes along a curve in which a vector end point indicating a tool direction is generated from an instructed vector sequence from the point sequence instructing a tool center point movement path and the vector sequence instructing the tool direction. JP-A 2009-146152 discloses a technique involving with a tool-direction instruction correction unit for correcting a tool-direction instruction so that a movement amount ratio between two rotation axes and a linear axis becomes uniform. Further, JP-A 2009-146152 represents a case where a tool direction does not proportionally change, but largely changes with respect to a length of a linear axis instruction as a tool center point movement path of a program created by CAM.
A position which is offset from the tool center point in the tool direction by the tool length is referred to as a control point. In a case where the tool center point is controlled so that the tool center point follows the instructed movement path, a change in the tool direction for each block is added to the control point while being amplified by the tool length. For this reason, in a case where a large change occurs in a tool-direction vector, the movement of the control point is not necessarily smooth even when the tool center point instruction path is smooth. When the control point movement path is not smooth, there is a need to abruptly accelerate or decelerate a servo motor so that the tool center point follows the path. As a result, a bad influence arises in that the processing time extends or the surface quality of the workpiece is degraded.
In the tool-direction instruction correction unit disclosed in JP-A 2009-146152, a rotation axis instruction is directly corrected. For this reason, it is not possible to control the correction direction of the tool-direction vector. For example, when the correction amount in the processing-surface normal direction increases in the processing using the tool side surface, the workpiece is cut more than needs, so that the surface quality of the workpiece is degraded. Further, when the correction amount increases in the direction with a barrier such as a machine, there is a possibility that the tool and the machine may interfere with each other as illustrated in FIG. 1.